LEDs have become the technology of choice for lighting products due to their high efficiency, low cost, compact size, rugged design and very long life. Unlike more conventional lighting technology, such as incandescent bulbs, fluorescent tubes and compact fluorescent lamps (CFLs), LEDs are current driven devices. Their light output is specified at a current through the LED 112. The voltage drop across an LED or LED string may vary from batch to batch by as much as 15 percent. Accordingly, it is common to drive an LED or LED string from a constant current circuit. A well-known circuit for this purpose is shown in FIG. 1, generally as 100. An input voltage is provided from an AC source 104 to a full wave rectifier 106. The output of the rectifier 106 is a DC voltage which is provided to a DC to DC converter 108 of an LED driver 102. An LED or LED string 112 is the load on the converter 108. A current sensor 114 is placed in series with the LED 112 to measure the current passing through the LED. The output of the current sensor 114 is applied to the inverting input of an error amplifier 116. The noninverting input of the amplifier 116 is connected to a current reference, such as voltage source 118. The output of the error amplifier 116 is coupled to a modulator 110, the output of which is coupled to the control input of the converter 108. Modulator 110 controls the operation of the converter 108 so that a predetermined current always flows through the LED 112 to produce a premeasured light output. The advantage of this circuit is an excellent line and load regulation of the output current. However, the circuit suffers from disadvantages, including poor power factor when operated from an AC input, a negative resistance input which is prone to oscillations in the presence of EMI filters, and poor compatibility with phase controlled dimmers. Phase controlled dimmers operate by clipping a portion of the AC waveform. As the voltage applied to the LED is reduced, the circuit attempts to maintain the current through the LED constant, thus interfering with the dimming. It is known to measure the angle of conduction, read the angle, and convert the value to change the current in the loop. This involves relatively complex circuitry.
A common problem in lighting is the variation in light intensity caused by voltage changes. For example, incandescent light bulbs suffer from a disadvantage that their light output varies widely when their operating voltage changes. This is because the power delivered to the bulb varies with the square of the operating voltage: if the operating voltage changes by 10 percent, the power delivered to the bulb changes by 21 percent. Furthermore, as the power to the bulb is diminished, the filament runs cooler—thus shifting more of the light into the invisible infrared spectrum (and vice-versa). This further exacerbates the flicker problem.
Therefore, there is a need for a simple and efficient device for driving an LED which at the same time minimizes the flicker problem.